Rates

One way to deal with proportional mixtures is to develop a language of ‘so-much-per-so-much’. This language is a useful link to composite measures such as density (so much mass per unit volume) or gradient (so much height per unit length). Speed is often the first rate that appears in the school curriculum, and is very hard to imagine, but early experiences of price-per-so-much and other unit ratios are also rates and can be used as bridging ideas.